Virtual reality can be viewed as a computer-generated simulated environment in which a user has an apparent physical presence. A virtual reality experience can be generated in 3D and viewed with a head-mounted display (HMD), such as glasses or other wearable display device that has near-eye display panels as lenses to display a virtual reality environment, which replaces the actual environment. Augmented reality, however, provides that a user can still see through the display lenses of the glasses or other wearable display device to view the surrounding environment, yet also see images of virtual objects that are generated for display and appear as a part of the environment. Augmented reality can include any type of input such as audio and haptic inputs, as well as virtual images, graphics, and video that enhances or augments the environment that a user experiences. As an emerging technology, there are many challenges and design constraints with augmented reality, from generation of the virtual objects and images so that they appear realistic in a real environment, to developing the optics small and precise enough for implementation with a wearable display device.
Some focusing systems for display technologies are typically too large and complex to be implemented in imaging units for wearable display devices. A mechanical mechanism to adjust focus is generally noisy, uses quite a lot of power, and is likely to be too slow to keep up with eye movement. Some focus adjustment may be accomplished with LCD lenses, but they are fixed focus and slower for higher and higher diopter powers. Another focusing system includes Alvarez lenses positioned on either side of a waveguide, where the first lens focuses the real object to focus infinity, the light is combined with the display light in the waveguide, and the second lens focuses back to the original viewing distance between the waveguide and the eye. However, the image quality of the lenses is limited and can induce distortion in the see-through and display light. Additionally, the lenses are relatively large, motors use a large amount of power (for a mobile device), and the mechanical mechanism to adjust focus is generally too slow to keep up with eye movement.
Focus accommodation for augmented reality near-eye displays is needed so that the conflict between stereopsis and focus accommodation cues is negated. However, current techniques for focus accommodation in waveguide displays are problematic, and proposed solutions are not ideal from mechanical, optical performance, and latency perspectives. With stereopsis, a virtual image that is projected or displayed for augmented reality imaging appears at focal infinity while a real object in the environment over which the virtual image is displayed appears closer. A viewer's eyes attempt to focus on the nearer, real object while also focusing at infinity even though the virtual image is projected very close to the viewer's eyes. This can cause viewer eye strain and fatigue, which may diminish the user experience when using a wearable display device.